Methods and intermediates for synthesizing SK1-I

ABSTRACT

The invention provides methods for synthesizing the compound (2R,3S,4E)-N-methyl-5-(4′-pentylphenyl)-2-aminopent-4-ene-1,3-diol, also known as SK1-I, and intermediate compounds used in its synthesis.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Ser.No. 62/576,943 filed Oct. 25, 2017 which is hereby incorporated byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the field of organic synthesis ofpharmaceutical compounds.

BACKGROUND OF THE INVENTION

(2R,3S,4E)-N-methyl-5-(4′-pentylphenyl)-2-aminopent-4-ene-1,3-diol, alsoknown as SK1-I and BML-258 (as HCl salt), is a pharmaceutical inhibitorof sphingosine kinase 1 initially described in Paugh et al., Blood. 2008Aug. 15; 112(4): 1382-1391. An existing method for synthesizing SK1-I isdisclosed in U.S. Pat. No. 8,314,151.

What is needed and provided by the present invention are improvedmethods for synthesizing SK1-I and related compounds.

SUMMARY OF THE INVENTION

One embodiment of the invention provides a method for synthesizing thecompound

that includes the reaction steps of:

and

Another embodiment of the invention provides a method for synthesizingthe compound

that includes the reaction step of:

A further embodiment of the invention provides a method for synthesizing

that includes the steps of:(i) reacting

with DIBAL in the presence of Rochelle salt,

thereby obtaining

In one variation of this embodiment, Rochelle salt is provided in anaqueous solution that further includes sodium hydroxide.

Still further embodiments of the invention provide complete multi-stepsyntheses of SK1-I and salts thereof from precursor compounds.

The invention also provides corresponding embodiments in which one ormore of the C₅H₁₁ alkyl group and the available phenyl group hydrogensare instead, independently, a linear or branched C₁₋₂₀ alkyl group orsuch a group having any subrange or number of carbons therein.

Additional features, advantages, and embodiments of the invention may beset forth or apparent from consideration of the following detaileddescription, drawings if any, and claims. Moreover, it is to beunderstood that both the foregoing summary of the invention and thefollowing detailed description are exemplary and intended to providefurther explanation without limiting the scope of the invention asclaimed.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods and intermediate compounds forsynthesizing the compound (2R,3S,4E)-N-methyl-5-(4′-pentylphenyl)-2-aminopent-4-ene-1,3-diol, alsoknown as SK1-I, and related compounds. The structure of SK1-I is shownbelow.

A step-wise synthesis of SK1-I according to the invention is exemplifiedas follows.

N-Boc-(D)-Serine Methyl Ester

To an ice-cooled suspension of the (D)-Serine methyl ester hydrochloride(62.24 g, 0.4 mol) in dichloromethane (600.0 mL), triethylamine (40.4 g,0.4 mol) was added. After the mixture was stirred for 30 min, Bocanhydride (96.0 g, 0.44 mol) in dichloromethane (100 mL) was addeddropwise with vigorous stirring over 30 min. The reaction mixture wasstirred for 16 hours at room temperature. Water (600 mL) was added. Theorganic layer was separated. The aqueous layer was extracted with 2×200mL of dichloromethane. The combined organic layer was washed with water(2×400 mL) and dried (Na₂SO₄). The solution was filtered, concentratedunder reduced pressure to give an oil 93.36 g (˜100% yield), which wasused directly in the next step without further purification.

Protection of N-Boc-(D)-Serine Methyl Ester

Boc-Serine methyl ester from above (93.0 g, 0.42 mol) and catalystp-toluenesulfonic acid (9.3 g) were dissolved in dichloromethane (500mL) and 2,2-dimethoxypropane (500 mL). The mixture was stirred at roomtemperature for 20 hours with a drying tube. Saturated sodiumbicarbonate (600.0 mL) was added. The mixture was then stirredvigorously for 30 min. The organic layer was separated, washed withbicarbonate (2×400.0 mL), water (400.0 mL), saturated NaCl (400.0 mL)and dried (Na₂SO₄). The solution was filtered and concentrated undervacuum to give 87.22 g oil (84% yield for two steps), which was useddirectly in the next step without further purification.

(R)—Garner Aldehyde

To a cooled solution of the ester (87.0 g, 0.336 mol) in anhydroustoluene (690.0 mL, −78° C., acetone/dry ice bath), DIBAL in toluene(1.49 M in toluene, 392 mL, 585.0 mmol) was added dropwise under argonin such a way that the internal temperature did not rise above −70° C.After the addition, the reaction mixture was stirred for an additional 4hours at −78° C. Methanol (128 mL) was added to the mixture to quenchthe reaction. The mixture was poured slowly into an aqueous solution ofRochelle salt (potassium sodium tartrate tetrahydrate; 1.2 M, 660 g/1949mL water) with vigorous stirring. The mixture was stirred at roomtemperature until clear separation into two layers. The aqueous layerwas extracted with diethyl ether (2×300.0 mL). The combined organiclayer was washed with water (2×800 mL) and brine (800 mL), then driedwith anhydrous Na₂SO₄. The solvent was evaporated under vacuum to givealdehyde as a pale yellow oil (68.59 g, 89%), which was used withoutfurther purification.

Addition of 4-Pentylphenyl Acetylene to the Above Aldehyde

To a cooled (−20° C.) solution of 4-n-pentylphenylacetylene (51.68 g,300 mmol) in dry THF (400 mL), n-BuLi solution (2.5 M in hexane, 120 mL,300 mmol) was added dropwise under argon. After 2 hours, the mixture wascooled to −78° C., followed by the addition of HMPA(hexmethylphosphoramide, 64.5 g, 360 mmol). After the mixture wasstirred at −78° C. for an additional 30 mins, methyl(R)-(+)-3-(t-butoxycarbonyl)-2,2-dimethyl-4-oxazolidinecarboxaldehyde(58.0 g, 248.3 mmol) in anhydrous THF (tetrahydrofuran; 100 mL) wasadded dropwise (maintaining the temperature below −60° C.). The mixturewas stirred for an additional 5 hours at −78° C., then quenched bysaturated ammonium chloride solution (1000 mL). The aqueous layer wasextracted with ethyl ether (3×400 mL). The combined organic layer waswashed with 0.5 N HCl (2×400 mL) and brine (400 mL), then dried withanhydrous sodium sulfate. The solvent was removed under vacuum to give ayellow oil (104.04 g, ˜100% yield), which was used without furtherpurification.

Deprotection of the Above Oxazolidine

To an ice cooled solution of Boc-oxazolidine (103.0 g, 257.0 mmol) inmethanol (1000 mL), was added conc. HCl (43.5 mL, pre-cooled to 0° C.).The mixture was stirred at room temperature overnight and then extractedwith hexane (3×400 mL). The pH of the methanol solution was adjustedwith solid sodium bicarbonate to 8.0. Boc anhydride (53.94 g, 245.92mmol) was added and the mixture was stirred at room temperature for 1-4hours until the disappearance of formed intermediate free amine. Thesolvent was removed under vacuum. The residue was redissolved in water(300 mL) and diethyl ether (300 mL). The ethyl ether layer was driedwith anhydrous sodium sulfate and then evaporated to give a brown oil(87.54 g, 94%), which was used without further purification.

Reduction of the Above Alcohol

To an ice-cooled solution of the above acetylene (87.0 g, 241.0 mmol) inTHF (800 mL), Red-Al (Sodium bis(2-methoxyethoxy)aluminum dihydride; 60%w/w in toluene, 392 mL; 1.205 mol) was added dropwise over 1 hour underargon with stirring. The solution was then stirred at room temperaturefor 36 hours. The reaction mixture was cooled in an ice bath and thenpoured carefully into a pre-cooled solution of Rochelle salt in water(700 g in 2200 mL of water). The mixture was vigorously stirred untiltwo layers were visible and well separated. The aqueous layer wasextracted with 2×600 mL of toluene. The combined toluene layer waswashed with water (2×800 mL) and saturated sodium chloride (800 mL) anddried (Na₂SO₄). The solvent was removed under vacuum to give a yellowishsemi solid, which was recrystallized with hexane (200 mL) to give awhite solid 43.3 g (purity: >98%; yield: 49%)

Deprotection to SK1-I (BML-258)

To a solution of Boc protected amine (15 g, 41.3 mmol) in anhydrous THF(300 mL), DIBAL (25% w/w in toluene, 1.49 M, 278 mL, 413 mmol) was addedat room temperature under argon. The mixture was refluxed until thestarting material disappeared. The mixture was cooled to roomtemperature and poured into Rochelle salt (340 g/1000 mL water)containing sodium hydroxide (50 g, ˜5%). The mixture was stirredvigorously for 1 hour. The aqueous layer was extracted with ethylacetate (2×500 mL). The combined organic layer was washed with water(1000 mL) and brine (1000 mL) and dried with anhydrous sodium sulfate.The solvent was removed under vacuum to afford yellowish oil, whichturned into a pale solid after storing at −20° C. overnight. To a coldsolution (ice bath) of this solid in ethyl ether (400 mL), was added 1MHCl in ethyl ether (50 mL). The white precipitate was collected byfiltration and washed with ethyl ether (2×50 mL), and then dried undervacuum to give product as a white solid (8.11 g, 63% yield).

Advantageous improvements obtained in this synthesis of SK1-I includethe following. First, introducing HMPA in the coupling of Garneraldehyde with acetylene improves the ratio of the desired erythro isomer(versus undesired threo-isomer) to ˜20:1 (from ˜8:1 without HMPA), thuseliminating the prior necessity of column purification to removeundesired isomer (threo-). Second, introducing Rochelle salt during thework up at the DIBAL reduction of Boc to a methyl group (last step)increased the yield of the pure product. And third, through a fewmodifications in different steps, the prior necessity for flash columnpurification was eliminated.

It should be understood that the synthesis described herein may bereadily adapted for any derivative in which the C₅H₁₁ group is replacedby a different alkyl group. For example, the invention providescorresponding embodiments in which the C₅H₁₁ alkyl group is instead alinear or branched C₁₋₂₀ alkyl group or such a group having any subrangeor number of carbons therein. The invention also provides correspondingembodiments in which one or more of the C₅H₁₁ alkyl group and theavailable hydrogen positions of the phenyl ring (together correspondingto positions R₁ through R₅ in the final compound generic formula printedbelow) independently have, instead of said group or hydrogenrespectively, a linear or branched C₁₋₂₀ alkyl group or such a grouphaving any subrange or number of carbons therein.

It should also be understood that in this disclosure and the appendedclaims where sequential reaction steps are graphically depicted that theoutput of a reaction step is the input of the next step. It should alsobe understood that wherever initial reactants are shown, such reactantsmay be provided in a providing step.

Any and all publications, patents, patent applications and otherdocuments cited in this application are hereby incorporated by referencein their entireties for all purposes to the same extent as if eachindividual publication, patent, patent application or other documentwere individually indicated to be incorporated by reference for allpurposes.

While various specific embodiments have been illustrated and described,it will be appreciated that various changes can be made withoutdeparting from the spirit and scope of the invention(s). Moreover,features described in connection with one embodiment of the inventionmay be used in conjunction with other embodiments, even if notexplicitly exemplified in combination within.

What is claimed is:
 1. A method for synthesizing the compound

comprising the reaction steps of:

(vii) reacting

with DIBAL in the presence of Rochelle salt, thereby obtaining